Self-Aligned Contact For Replacement Gate Devices

ABSTRACT

A conductive top surface of a replacement gate stack is recessed relative to a top surface of a planarization dielectric layer by at least one etch. A dielectric capping layer is deposited over the planarization dielectric layer and the top surface of the replacement gate stack so that the top surface of a portion of the dielectric capping layer over the replacement gate stack is vertically recessed relative to another portion of the dielectric layer above the planarization dielectric layer. The vertical offset of the dielectric capping layer can be employed in conjunction with selective via etch processes to form a self-aligned contact structure.

BACKGROUND

The present disclosure relates to semiconductor structures, andparticularly to a metal-oxide-semiconductor field effect transistor(MOSFET) having a self-aligned contact structure and methods ofmanufacturing the same.

As semiconductor devices shrink in each generation of semiconductortechnology, formation of contact structures to source and drain regionsof a field effect transistor become challenging because such contactstructures not only need to provide reliable electrical contact to thesource and drain regions, but also need to avoid electrically shortingto other components such as the gate electrode of the field effecttransistor. Since the etch chemistry employed for the anisotropic etchprocess remains the same while the lateral dimension of the dielectricgate spacer shrinks with the scaling of semiconductor devices, thelikelihood of overlay variations during lithographic processes causingformation of contact structures that electrically short a source/drainregion to a gate conductor of a field effect transistor increases ineach generation.

Nonetheless, contact structures to source and drain regions must avoidelectrically shorting to gate conductors to provide a functional fieldeffect transistor. Thus, the possibility of electrically shortingsource/drain regions to a gate conductor of a field effect transistor isa significant concern for product yield and reliability purposes.

BRIEF SUMMARY

A conductive top surface of a replacement gate stack is recessedrelative to a top surface of a planarization dielectric layer by atleast one etch. A dielectric capping layer is deposited over theplanarization dielectric layer and the top surface of the replacementgate stack so that the top surface of a portion of the dielectriccapping layer over the replacement gate stack is vertically recessedrelative to another portion of the dielectric layer above theplanarization dielectric layer. The vertical offset of the dielectriccapping layer can be employed in conjunction with selective via etchprocesses to form a self-aligned contact structure.

According to an aspect of the present disclosure, a method of forming asemiconductor structure is provided, which includes: forming a gateelectrode and a planarization dielectric layer on a semiconductorsubstrate, wherein a top metallic surface of the gate electrode iscoplanar with a top surface of the planarization dielectric layer;recessing a top surface of the gate electrode relative to the topsurface of the planarization dielectric layer; forming an etch stoplayer contiguously on the recessed top surface of the gate electrode andon the top surface of the planarization dielectric layer, wherein theetch stop layer includes a first portion located on the planarizationdielectric layer and a second portion located on the gate electrode andhaving a vertically offset bottom surface relative to an interfacebetween the planarization dielectric layer and the first portion;forming a contact-level dielectric layer over the etch stop layer; andforming a via hole extending at least through the contact-leveldielectric layer, the first portion of the etch stop layer, and aportion of the planarization dielectric layer, wherein the via hole isvertically spaced from the gate electrode by the second portion of theetch stop layer.

According to another aspect of the present disclosure, a semiconductorstructure is provided, which includes: a gate electrode located on asemiconductor substrate; a planarization dielectric layer laterallysurrounding the gate electrode, wherein a top surface of the gateelectrode is recessed relative to a top surface of the planarizationdielectric layer; an etch stop layer contiguously located on therecessed top surface of the gate electrode and the top surface of theplanarization dielectric layer, wherein the etch stop layer includes afirst portion located on the planarization dielectric layer and a secondportion located on the gate electrode and having a vertically offsetbottom surface relative to an interface between the planarizationdielectric layer and the first portion; a contact-level dielectric layeroverlying the planarization dielectric layer; and a contact viastructure extending through the contact-level dielectric layer, the etchstop layer, and the planarization dielectric layer, wherein a portion ofthe contact via structure overlies the gate electrode and is verticallyspaced by the second portion of the etch stop layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is vertical cross-sectional view of an exemplary semiconductorstructure after formation of disposable gate structures and formation ofa planar dielectric surface on a planarization dielectric layeraccording to an embodiment of the present disclosure.

FIG. 2 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 1 after removal of the disposable gate structures.

FIG. 3 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 2 after formation of a first-type work functionmaterial layer.

FIG. 4 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 3 after application of a photoresist and lithographicpatterning of the first-type work function material layer.

FIG. 5 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 4 after removal of the photoresist and formation of asecond-type work function material layer.

FIG. 6 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 5 after deposition of at least one metallic barrierlayer and a conductive metal layer.

FIG. 7 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 6 after planarization.

FIG. 8 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 7 after recessing an upper portion of a gate electrodeby at least one wet etch.

FIG. 9 is a vertical cross-sectional view of the exemplary semiconductorstructure of FIG. 8 after removing protruding portions of gateelectrodes by a dry etch.

FIG. 10 is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 9 after formation of an etch stopdielectric layer and a contact-level dielectric layer.

FIG. 11 is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 10 during formation of self-aligned viaholes.

FIG. 12 is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 11 after formation of the self-alignedvia holes.

FIG. 13 is a vertical cross-sectional view of the exemplarysemiconductor structure of FIG. 12 after formation of self-alignedcontact via structures.

FIG. 14 is a vertical cross-sectional view of an alternate exemplarysemiconductor structure after formation of self-aligned contact viastructures.

DETAILED DESCRIPTION

As stated above, the present disclosure, which relates to semiconductorstructures having dual work function metal gates and a high-k gatedielectric, and methods of manufacturing the same, will now be describedin detail with accompanying figures. Like and corresponding elementsmentioned herein and illustrated in the drawings are referred to by likereference numerals. The drawings are not necessarily drawn to scale.

Referring to FIG. 1, an exemplary semiconductor structure according to afirst embodiment of the present disclosure includes a semiconductorsubstrate 8, on which various components of field effect transistors areformed. The semiconductor substrate 8 can be a bulk substrate includinga bulk semiconductor material throughout, or asemiconductor-on-insulator (SOI) substrate (not shown) containing a topsemiconductor layer, a buried insulator layer located under the topsemiconductor layer, and a bottom semiconductor layer located under theburied insulator layer.

Various portions of the semiconductor material in the semiconductorsubstrate 8 can be doped with electrical dopants of p-type or n-type atdifferent dopant concentration levels. For example, the semiconductorsubstrate 8 may include an underlying semiconductor layer 10, a firstconductivity type well 12B, and a second-conductivity type well 12A. Thefirst conductivity type well 12B is doped with electrical dopants of afirst conductivity type, which can be p-type or n-type. The secondconductivity type well 12A is doped with electrical dopants of a secondconductivity type, which is the opposite type of the first conductivitytype. For example, if the first conductivity type is p-type, the secondconductivity type is n-type, and vice versa.

Shallow trench isolation structures 20 are formed to laterally separateeach of the first conductivity type well 12B and the second conductivitytype well 12A. Typically, each of the first conductivity type well 12Band the second conductivity type well 12A is laterally surrounded by acontiguous portion of the shallow trench isolation structures 20. If thesemiconductor substrate 8 is a semiconductor-on-insulator substrate,bottom surfaces of the first conductivity type well 12B and the secondconductivity type well 12A may contact a buried insulator layer (notshown), which electrically isolates each of the first conductivity typewell 12B and the second conductivity type well 12A from othersemiconductor portions of the semiconductor substrate 8 in conjunctionwith the shallow trench isolation structures 20.

A disposable dielectric layer and a disposable gate material layer aredeposited and lithographically patterned to form disposable gatestructures. For example, the disposable gate structures may include afirst disposable gate structure that is a stack of a first disposabledielectric portion 29A and a first disposable gate material portion 27Aand a second disposable gate structure that is a stack of a seconddisposable dielectric portion 29B and a second disposable gate materialportion 27B. The disposable dielectric layer includes a dielectricmaterial such as a semiconductor oxide. The disposable gate materiallayer includes a material that can be subsequently removed selective todielectric material such as a semiconductor material. The firstdisposable gate structure (29A, 27A) is formed over the secondconductivity type well 12A, and the second disposable gate structure(29B, 27B) is formed over the first conductivity type well 12B. Theheight of the first disposable gate structure (29A, 27A) and the seconddisposable gate structure (29B, 27B) can be from 20 nm to 500 nm, andtypically from 40 nm to 250 nm, although lesser and greater heights canalso be employed.

Dopants of the first conductivity type are implanted into portions ofthe second conductivity type well 12A that are not covered by the firstdisposable gate structure (29A, 27A) to form first source and drainextension regions 14A. The first conductivity type well 12B can bemasked by a photoresist (not shown) during the implantation of the firstconductivity type dopants to prevent implantation of the firstconductivity type dopants therein. Similarly, dopants of the secondconductivity type are implanted into portions of the first conductivitytype well 12B that are not covered by the second disposable gatestructure (29B, 27B) to form second source and drain extension regions14B. The second conductivity type well 12A can be masked by aphotoresist (not shown) during the implantation of the secondconductivity type dopants to prevent implantation of the secondconductivity type dopants therein.

Dielectric gate spacers are formed on sidewalls of each of thedisposable gate structures, for example, by deposition of a conformaldielectric material layer and an anisotropic etch. The dielectric gatespacers include a first dielectric gate spacer 52A formed around thefirst disposable gate structure (29A, 27A) and a second dielectric gatespacer 52B formed around the second disposable gate structure (29B,27B).

Dopants of the first conductivity type are implanted into portions ofthe second conductivity type well 12A that are not covered by the firstdisposable gate structure (29A, 27A) and the first dielectric gatespacer 52A to form first source and drain regions 16A. The firstconductivity type well 12B can be masked by a photoresist (not shown)during the implantation of the first conductivity type dopants toprevent implantation of the first conductivity type dopants therein.Similarly, dopants of the second conductivity type are implanted intoportions of the first conductivity type well 12B that are not covered bythe second disposable gate structure (29B, 27B) and the seconddielectric gate spacer 52B to form second source and drain regions 16B.The second conductivity type well 12A can be masked by a photoresist(not shown) during the implantation of the second conductivity typedopants to prevent implantation of the second conductivity type dopantstherein.

In some embodiments, the first source and drain regions 16A and/or thesecond source and drain regions 16B can be formed by replacement of thesemiconductor material in the second conductivity type well 12A and/orthe semiconductor material in the first conductivity type well 12B witha new semiconductor material having a different lattice constant. Inthis case, the new semiconductor material(s) is/are typicallyepitaxially aligned with (a) single crystalline semiconductormaterial(s) of the second conductivity type well 12A and/or thesemiconductor material in the first conductivity type well 12B, andapply/applies a compressive stress or a tensile stress to thesemiconductor material of the second conductivity type well 12A and/orthe semiconductor material in the first conductivity type well 12Bbetween the first source and drain extension regions 14A and/or betweenthe second source and drain extension regions 14B.

First metal semiconductor alloy portions 46A and second metalsemiconductor alloy portions 46B are formed on exposed semiconductormaterial on the top surface of the semiconductor substrate 8, forexample, by deposition of a metal layer (not shown) and an anneal.Unreacted portions of the metal layer are removed selective to reactedportions of the metal layer. The reacted portions of the metal layerconstitute the metal semiconductor alloy portions (46A, 46B), which caninclude a metal silicide portions if the semiconductor material of thefirst and second source and drain regions (16A, 16B) include silicon.

Optionally, a dielectric liner 54 may be deposited over the metalsemiconductor alloy portions 54, the first and second disposable gatestructures (29A, 27A, 29B, 27B), and the first and second dielectricgate spacers (52A, 52B). The dielectric liner 54 can include adielectric material such as silicon oxide, silicon oxynitride, siliconnitride, or a combination thereof. A first type stress-generating liner58 and a second type stress-generating liner 56 can be formed over thefirst disposable gate structure (29A, 27A) and the second disposablegate structure (29B, 27B), respectively. The first typestress-generating liner 58 and/or the second type stress-generatingliner 56 can be employed to apply uniaxial or biaxial lateral stress toa first channel region, which is the portion of the second conductivitytype well 12A between the first source and drain extension regions 14A,and/or to a second channel region, which is the portion of the firstconductivity type well 12B between the second source and drain extensionregions 14B, respectively. In one embodiment, one of the first typestress-generating liner 58 and the second type stress-generating liner56 applies a compressive stress if underlying source and drain regions(i.e., the first source and drain regions 16A or the second source anddrain regions 16B) are p-doped regions, and the other of the first typestress-generating liner 58 or the second type stress-generating liner 56applies a tensile stress if underlying source and drain regions (i.e.,the second source and drain regions 16B and the first source and drainregions 16A) are n-doped regions. The first type stress-generating liner58 and the second type stress-generating liner 56 can include adielectric material that generates a compressive stress or a tensilestress to underlying structures, and can be silicon nitride layersdeposited by plasma enhanced chemical vapor deposition under variousplasma conditions.

A planarization dielectric layer 60 is deposited over the first typestress-generating liner 58 and/or the second type stress-generatingliner 56, if present, or over the metal semiconductor alloy portions 54,the first and second disposable gate structures (29A, 27A, 29B, 27B),and the first and second dielectric gate spacers (52A, 52B) if (a)stress-generating liner(s) is/are not present. The planarizationdielectric layer 60 can be formed by depositing a dielectric material,which can be selected from undoped silicate glass (USG), doped silicateglass, silicon nitride, organosilicate glass (OSG), undoped siliconoxynitride, and doped silicon oxynitride. Preferably, the planarizationdielectric layer 60 is a dielectric material that may be easilyplanarized. For example, the planarization dielectric layer 60 can be adoped silicate glass or an undoped silicate glass (silicon oxide).

The planarization dielectric layer 60, the first type stress-generatingliner 58 and/or the second type stress-generating liner 56 (if present),and the dielectric liner 54 (if present) are planarized above thetopmost surfaces of the first and second disposable gate structures(29A, 27A, 29B, 27B), i.e., above the topmost surfaces of the first andsecond disposable gate material portions (27A, 27B). The planarizationcan be performed, for example, by chemical mechanical planarization(CMP). The planar topmost surface of the planarization dielectric layer60 is herein referred to as a planar dielectric surface 63.

In one embodiment, the first conductivity type is p-type and the secondconductivity type is n-type. The first source and drain extensionregions 14A and the first source and drain regions 16A are p-doped, andthe second conductivity type well 12A is n-doped. The combination of thefirst source and drain extension regions 14A, the first source and drainregions 16A, and the second conductivity type well 12A can be employedto subsequently form a p-type field effect transistor. Correspondingly,the first source and drain extension regions 14A and the first sourceand drain regions 16A are n-doped, and the second conductivity type well12A is p-doped. The combination of the first source and drain extensionregions 14A, the first source and drain regions 16A, and the secondconductivity type well 12A can be employed to subsequently form ansecond field effect transistor. The first type stress-generating liner58 can apply a tensile stress to the first channel, and the second typestress-generating liner 56 can apply a compressive stress to the secondchannel.

In another embodiment, the first conductivity type is n-type and thesecond conductivity type is p-type. The first source and drain extensionregions 14A and the first source and drain regions 16A are n-doped, andthe second conductivity type well 12A is p-doped. The combination of thefirst source and drain extension regions 14A, the first source and drainregions 16A, and the second conductivity type well 12A can be employedto subsequently form an n-type field effect transistor. Correspondingly,the first source and drain extension regions 14A and the first sourceand drain regions 16A are p-doped, and the second conductivity type well12A is n-doped. The combination of the first source and drain extensionregions 14A, the first source and drain regions 16A, and the secondconductivity type well 12A can be employed to subsequently form a firstfield effect transistor. The first type stress-generating liner 58 canapply a compressive stress to the first channel, and the second typestress-generating liner 56 can apply a tensile stress to the secondchannel.

Referring to FIG. 2, the first disposable gate structure (29A, 27A) andthe second disposable gate structure (29B, 27B) are removed by at leastone etch. The at least one etch can be a recess etch, which can be anisotropic etch or anisotropic etch. The etch employed to remove thefirst and second disposable gate material portions (27A, 27B) ispreferably selective to the dielectric materials of the planarizationdielectric layer 60, the first type stress-generating liner 58 and/orthe second type stress-generating liner 56 (if present), and the firstand second dielectric gate spacers (52A, 52B). Optionally, one or bothof the dielectric portions (29A, 29B) can be left by etch selective tothese layers. The disposable gate structures (29A, 27A, 29B, 27B) arerecessed below the planar dielectric surface 63 and to expose thesemiconductor surfaces above the first channel and the second channel toform gate cavities (25A, 25B) over the semiconductor substrate 8.

Optionally, a first semiconductor-element-containing dielectric layer31A can be formed on the exposed surface of the second conductivity typewell 12A by conversion of the exposed semiconductor material into adielectric material, and a second semiconductor-element-containingdielectric layer 31B can be formed on the exposed surface of the firstconductivity type well 12B by conversion of the exposed semiconductormaterial into the dielectric material. The formation of thesemiconductor-element-containing dielectric layers (31A, 31B) can beeffected by thermal conversion or plasma treatment. If the semiconductormaterial of the second conductivity type well 12A and the firstconductivity type well 12B includes silicon, thesemiconductor-element-containing dielectric layers (31A, 31B) caninclude silicon oxide or silicon nitride. Thesemiconductor-element-containing dielectric layers (31A, 31B) areinterfacial dielectric layers that contact a semiconductor surfaceunderneath and gate dielectrics to be subsequently deposited thereupon.

Referring to FIG. 3, a contiguous gate dielectric layer 32L and afirst-type work function metal layer 34L including a first metal havinga first work function are sequentially formed for form a stack, frombottom to top, of the contiguous gate dielectric layer 32L and thefirst-type work function metal layer 34L. The contiguous gate dielectriclayer 32L can be a high dielectric constant (high-k) material layerhaving a dielectric constant greater than 8.0. The contiguous gatedielectric layer 32L can include a dielectric metal oxide, which is ahigh-k material containing a metal and oxygen, and is known in the artas high-k gate dielectric materials. Dielectric metal oxides can bedeposited by methods well known in the art including, for example,chemical vapor deposition (CVD), physical vapor deposition (PVD),molecular beam deposition (MBD), pulsed laser deposition (PLD), liquidsource misted chemical deposition (LSMCD), atomic layer deposition(ALD), etc. Exemplary high-k dielectric material include HfO₂, ZrO₂,La₂O₃, Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y),La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y),LaAlO_(x)N_(y), Y₂O_(x)N_(y), a silicate thereof, and an alloy thereof.Each value of x is independently from 0.5 to 3 and each value of y isindependently from 0 to 2. The thickness of the contiguous gatedielectric layer 32L, as measured at horizontal portions, can be from0.9 nm to 6 nm, and preferably from 1.0 nm to 3 nm. The high-k materiallayer 32L may have an effective oxide thickness on the order of or lessthan 1 nm.

The first-type work function metal layer 34L includes a first metal,which has a first work function. The first metal of the first-type workfunction metal layer 34L is selected to optimize the performance of atransistor to be subsequently formed employing the first source anddrain extension regions 14A, the first source and drain regions 16A, andthe second conductivity type well 12A.

In one embodiment, the first conductivity type is p-type and thesemiconductor material of the second conductivity type well 12A includesn-doped silicon, and the first-type work function metal layer 34Lincludes a silicon valence band edge metals such as Pt, Rh, Ir, Ru, Cu,Os, Be, Co, Pd, Te, Cr, Ni, TiN, and alloys thereof. A silicon valenceband edge metal is a metal having a work function that is closer to thework function corresponding to the valence band edge of silicon, i.e.,5.10 eV, than to the work function corresponding to the conduction bandedge of silicon, i.e., 4.00 eV. Thus, a silicon valence band edge metalhas a work function that is greater than 4.55 eV. For example, thefirst-type work function metal layer 34L can be a layer of TiN.

In another embodiment, the first conductivity type is n-type and thesemiconductor material of the second conductivity type well 12A includesp-doped silicon, and the first-type work function metal layer 34Lincludes a silicon conduction band edge metals such as Hf, Ti, Zr, Cd,La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg,Gd, Y, and TiAl, and alloys thereof. A silicon conduction band edgemetal is a metal having a work function that is closer to the workfunction corresponding to the conduction band edge of silicon than tothe work function corresponding to the valence band edge of silicon.Thus, a silicon conduction band edge metal has a work function that isless than 4.55 eV. For example, the first-type work function metal layer34L can be a layer of TiAl.

The first-type work function metal layer 34L can be formed, for example,by physical vapor deposition, chemical vapor deposition, or atomic layerdeposition (ALD). The thickness of the first-type work function metallayer 34L is typically set at a value from 1 nm to 30 nm, and moretypically, from 2 nm to 10 nm, although lesser and greater thicknessescan also be employed.

Referring to FIG. 4, a first photoresist 39 is applied and lithographicpatterned so that the first photoresist 39 covers the area over thesecond conductivity type well 12A, while the top surface of thefirst-type work function metal layer 34L is exposed over the firstconductivity type well 12B. The pattern in the first photoresist 39 istransferred into the first-type work function metal layer 34L by anetch. The portion of the first-type work function metal layer 34L withinthe second gate cavity 25B is removed employing the first photoresist 39as an etch mask. The first photoresist 39 is removed, for example, byashing or wet etching. After the patterning of the first-type workfunction metal layer 34L, the first-type work function metal layer 34Lis present in the first gate cavity 25A (See FIG. 4), but is not presentin the second gate cavity 25B.

Referring to FIG. 5, a second-type work function metal layer 36L isdeposited. The second-type work function metal layer 36L includes asecond metal having a second work function, which is different from thefirst work function. The second metal of the second-type work functionmetal layer 36L is selected to optimize the performance of a transistorto be subsequently formed employing the second source and drainextension regions 14B, the second source and drain regions 16B, and thefirst conductivity type well 12B.

In one embodiment, the second conductivity type is n-type and thesemiconductor material of the first conductivity type well 12B includesp-doped silicon, and the second-type work function metal layer 36Lincludes a silicon conduction band edge metals such as Hf, Ti, Zr, Cd,La, Tl, Yb, Al, Ce, Eu, Li, Pb, Tb, Bi, In, Lu, Nb, Sm, V, Zr, Ga, Mg,Gd, Y, and TiAl, and alloys thereof. For example, the second-type workfunction metal layer 36L can be a layer of TiAl.

In another embodiment, the second conductivity type is p-type and thesemiconductor material of the first conductivity type well 12B includesn-doped silicon, and the second-type work function metal layer 36Lincludes a silicon valence band edge metals such as Pt, Rh, Ir, Ru, Cu,Os, Be, Co, Pd, Te, Cr, Ni, TiN, and alloys thereof. For example, thesecond-type work function metal layer 36L can be a layer of TiN.

The second-type work function metal layer 36L can be formed, forexample, by physical vapor deposition, chemical vapor deposition, oratomic layer deposition (ALD). The thickness of the second-type workfunction metal layer 34L is typically set at a value from 2 nm to 100nm, and more typically, from 3 nm to 10 nm, although lesser and greaterthicknesses can also be employed.

In general, one of the first and second work functions is closer to theconduction band of the semiconductor material of the first conductivitytype well 12B and the second conductivity type well 12A than the valenceband of that semiconductor material, and the other of the first andsecond work functions is closer to the valence band than to theconduction band of that material. Typically, the work function that iscloser to the conduction band than to the valence band of thesemiconductor material is employed to enhance the performance of ann-type field effect transistor, and the work function that is closer tothe valence band than to the conduction band of the semiconductormaterial is employed to enhance the performance of a p-type field effecttransistor.

Referring to FIG. 6, an optional barrier metal layer 38L can depositedon the second-type work function metal layer 36L. In a non-limitingillustrative example, the optional barrier metal layer 38L can include atantalum nitride layer, a titanium nitride layer, a titanium-aluminumalloy, or a combination thereof. The thickness of the optional barriermetal layer 38L can be from 0.5 nm to 20 nm, although lesser and greaterthicknesses can also be employed. The optional barrier metal layer 38Lmay be omitted in some embodiments. In one embodiment, the optionalbarrier metal layer 38L includes a metallic nitride. For example, theoptional barrier metal layer 38L can include titanium nitride.

A conductive metal layer 40L is deposited on the optional barrier metallayer 38L or on the second-type work function metal layer 36L. Theconductive metal layer 40L can include a conductive material depositedby physical vapor deposition or chemical vapor deposition. For example,the conductive metal layer 40L can be an aluminum layer or an aluminumalloy layer deposited by physical vapor deposition. The thickness of theconductive metal layer 40L, as measured in a planar region of theconductive metal layer 40L above the top surface of the planarizationdielectric layer 60, can be from 100 nm to 500 nm, although lesser andgreater thicknesses can also be employed. In one embodiment, theconductive metal layer 40 consists essentially of a single elementalmetal such as Al, Au, Ag, Cu, or W. For example, the conductive metallayer can consist essentially of aluminum.

At the end of the processing step of FIG. 6, a material layer stack isformed, which includes, from bottom to top, the contiguous gatedielectric layer 32L which is a gate dielectric layer, the first-typework function metal layer 34L, the second-type work function metal layer36L, the optional barrier metal layer 38L, if present, and theconductive metal layer 40L. The portion of the material layer stack inthe first device region includes the first-type work function metallayer 34L and a portion of the second-type work function metal layer36L. The portion of the material layer stack in the second device regionincludes another portion of the second-type work function metal layer36L, but does not include any portion of the first-type work functionmetal layer 34L. The material layer stack is present over the topsurface of the planarization dielectric layer 60 at this step.

Referring to FIG. 7, portions of the gate conductor layer 40L, theoptional barrier metal layer 38L, the second-type work function metallayer 36L, the first-type work function metal layer 34L, and thecontiguous gate dielectric layer 32L are removed from above the planardielectric surface 63 of the planarization dielectric layer 60 byemploying a planarization process. Replacement gate stacks are formed byremoving portions of the material layer stack from above a source regionand a drain region of each field effect transistor. The replacement gatestacks include a first replacement gate stack 230A located in the firstdevice region and a second replacement gate stack 230B located in thesecond device region. Each replacement gate stack (230A, 230B) overliesa channel region of a field effect transistor. The first replacementgate stack 230A and the second replacement gate stack 230B are formedconcurrently.

A first field effect transistor is formed in the first device region.The first field effect transistor includes the p-type well 12A, thefirst source and drain extension regions 14A, the first source and drainregions 16A, first metal semiconductor alloy portions 46A, and a firstreplacement gate stack 230A. The first replacement gate stack 230Aincludes the optional first semiconductor-element-containing dielectriclayer 31A, a first U-shaped gate dielectric 32A which is a remainingportion of the contiguous gate dielectric layer 32L in the first deviceregion, and a first gate electrode. The first gate electrode includes afirst-type work function metal portion 34 which is a remaining portionof the first-type work function metal layer 34L, a metallic materialportion 36A which is a remaining portion of the second-type workfunction metal layer 36L in the first device region, a first optionalbarrier metal portion 38A which is a remaining portion of the optionalbarrier metal layer 38L, and a first gate conductor portion 40A which isa remaining portion of the gate conductor layer 40L.

A second field effect transistor is formed in the second device region.The second field effect transistor includes the n-type well 12B, then-type source and drain extension regions 14B, the n-type source anddrain regions 16B, a second metal semiconductor alloy portions 46B, anda second replacement gate stack 230B. The second replacement gate stack230B includes the optional second semiconductor-element-containingdielectric layer 31B, a second U-shaped gate dielectric 32B which is aremaining portion of the contiguous gate dielectric layer 32L in thesecond device region, and a second gate electrode. The second gateelectrode includes a second-type work function metal portion 36B whichis a remaining portion of the second-type work function metal layer 36Lin the second device region, a second optional barrier metal portion 38Bwhich is a remaining portion of the optional barrier metal layer 38L,and a second gate conductor portion 40B which is a remaining portion ofthe gate conductor layer 40L. The metallic material portion 36A in thefirst replacement gate stack 230A and the second-type work functionmetal portion 36B in the second replacement gate stack 230B have thesame material composition and the same thickness.

Each of the first and second U-shaped gate dielectrics (32A, 32B) has adielectric constant greater than 8.0. Each of the first and secondU-shaped gate dielectrics (32A, 32B) includes a horizontal gatedielectric portion and a vertical gate dielectric portion extendingupward from peripheral regions of the horizontal gate dielectricportion. In the first field effect transistor, the first-type workfunction metal portion 34 contacts inner sidewalls of the vertical gatedielectric portion of the first U-shaped gate dielectric 32A. In thesecond field effect transistor, the second-type work function metalportion 36B contacts inner sidewalls of the vertical gate dielectricportion of the second U-shaped gate dielectric 32B.

All top metallic surfaces of each of the first gate electrode 220A andthe second gate electrode 220B can be coplanar with the top surface ofthe planarization dielectric layer 60, i.e., the planar dielectricsurface 63. The disposable gate structures are replaced with the firstand second replacement gate structures (230A, 230B) each including agate dielectric (32A, 32B) and a gate electrode including at least oneconductive material portion. Each gate electrode is a remaining portionof the at least one conductive material after removing the at least oneconductive material from above a top surface of the planarizationdielectric layer. A dielectric gate spacer (52A or 52B) laterallysurrounds each gate electrode.

Referring to FIG. 8, the upper portions of the first gate electrode 220Aand the second gate electrode 220B are exposed to at least one etch sothat the top surfaces of the first gate electrode 220A and the secondgate electrode 220B are recessed relative to the top surface of theplanarization dielectric layer 60. The at least one etch can include oneor more wet etches only, one or more dry etches only, or a combinationof at least one wet etch and at least one dry etch. As used herein, a“wet etch” is an etch in which an etchant liquid is applied to amaterial to be etched, and a “dry etch” is an etch in which a gas phaseetchant or a plasma phase etchant is applied to a material to be etched.

At least one wet etch can be employed to remove one or more conductivematerial from the first gate electrode 220A and the second gateelectrode 220B. In an exemplary illustration, the at least oneconductive material in each of the first gate electrode 220A and thesecond gate electrode 220B can include a titanium nitride portion and analuminum portion. For example, if the second-type work function metallayer 36L includes titanium nitride, the metallic material portion 36Ain the first replacement gate structure 230A and the second-type workfunction metal portion 36B in the second replacement gate structure 230Balso include titanium nitride. In this case, a wet etch solutionincluding ammonium hydroxide and hydrogen peroxide can be employed toetch the titanium nitride portions, viz., the metallic material portion36A and the second-type work function metal portion 36B. The wet etchsolution including ammonium hydroxide and hydrogen peroxide may, or maynot, be diluted in water.

In another example, the gate conductor layer 40L can be an aluminumlayer, and each of the first gate conductor portion 40A and the secondgate conductor portion 40B can be an aluminum portion. In this case, thefirst gate conductor portion 40A and the second gate conductor portion40B can be etched by another wet etch that employs a sulfuricperoxide-containing solution and/or a dilute hydrofluoricacid-containing solution. Specifically, the wet etch can employ asolution including at least sulfuric acid and hydrogen peroxide or adilute hydrofluoric acid-containing solution. The sulfuricperoxide-containing solution can be a diluted sulfuric peroxide solutionor a mixture of a diluted sulfuric peroxide solution and hydrofluoricacid at a concentration between 1 ppm and 100 ppm or a mixture, dilutedin water, of sulfuric peroxide solution, hydrogen peroxide, andhydrofluoric acid at a concentration between 1 ppm and 100 ppm.

In one embodiment, the upper portions of the at least one conductivematerial of the first gate electrode 220A and the second gate electrode220B can be recessed by a combination of a wet etch and a dry etch. Forexample, if a wet etch or a combination of wet etches does not effectiveremove all types of conductive materials in the first gate electrode220A and the second gate electrode 220B, one or more dry etch processcan be employed to remove any protruding portion of the first gateelectrode 220A and the second gate electrode 220B. For example, tantalumnitride portions are known to be difficult to remove employing wet etchchemistries. Thus, if any of the conductive material portions in thefirst gate electrode 220A and the second gate electrode 220B is atantalum nitride portion, such a tantalum nitride portion may protrudeabove recessed top surfaces of other conductive materials after one ormore wet etches are performed. This situation is illustrated in FIG. 8for a non-limiting illustrative case in which the first-type workfunction metal portion 34 is a tantalum nitride portion or a metallicmaterial portion including a metallic material that cannot be easilyetched employing wet etch chemistries.

Referring to FIG. 9, a dry etch can be employed to remove any protrudingconductive material portion of the first gate electrode 220A and thesecond gate electrode 220B that is not recessed by one or more wetetches. Such a protruding conductive material portion, which can be atantalum nitride portion or a titanium nitride portion, can be removedby a dry etch. In one embodiment, plasma damage to gate dielectrics(e.g., the first gate dielectric (31A, 32A) and the second gatedielectric (32B, 32B)) can be avoided by employing a dry etch that doesnot apply plasma to the first and second replacement gate stacks (230A,230B). For example, one or more protruding conductive material portionin the first gate electrode 220A or the second gate electrode 220B canbe removed by a chemical downstream etch that does not apply plasma tothe gate stack.

A chemical downstream etch (CDE) differs from reactive ion etch (RIE) orplasma etch (PE) in that chemical downstream etch performs an etchisotropically, i.e., at the same etch rate in all directions. In achemical downstream etch, a plasma is generated in a radical generatorlocated outside a process chamber in which the exemplary semiconductorstructure is placed. Only radicals can get out of this radical generatorwhile the ions remain trapped inside the radical generator. Theprocesses gases are dissociated in the radical generator and distributedequally in the process chamber in which the exemplary semiconductorstructure is present. This geometry causes equal removal from allsurfaces exposed to the radicals, thereby causing isotropic etching.Thus, protruding conductive material portions in the first gateelectrode 220A and the second gate electrode 220B can be removed by achemical downstream etch isotropically. A chemical downstream etch canremove metal oxides that are difficult to remove by wet etches. Etchantgases that can be used for CDE include, but are not limited to, CHF₃ andCF₄, which are especially effective for metals such as Ta or Ti. Cl₂ gascan also be employed as an etchant gas in a CDE process. Specifically,the at least one conductive material in the one or more protrudingconductive material portion in the first gate electrode 220A or thesecond gate electrode 220B can include at least a tantalum nitrideportion or a titanium nitride portion. The CDE can etch at least onetantalum nitride portion or at least one titanium nitride portion. TheCDE can employ at least one of CHF₃, CF₄, and Cl₂ and does not generateplasma in the process chamber, and consequently does not generate anyplasma directly on any gate stack, and thereby preventing plasma damageof gate dielectric materials.

The combination of the at least one wet etch and at least one dry etchcan be employed to recess all conductive top surfaces of the first gateelectrode 220A and the second gate electrode 220B. Substantially flatrecessed top surfaces can be formed on the first gate electrode 220A andthe second gate electrode 220B. After removing all protruding portionsof gate electrodes by at least one dry etch, top surfaces of the firstgate electrode 220A and the second gate electrode 220B are recessedrelative to the top surface of the planarization dielectric layer 60.The depth of the recess, i.e., the recess depth rd, can be from 3 nm to30 nm, and typically, from 5 nm to 10 nm, although lesser and greaterrecess depths can also be employed.

In one embodiment, the exposed portions of the firstsemiconductor-element-containing dielectric layer 31A can be formed onthe exposed surface of the second conductivity type well 12A byconversion of the exposed semiconductor material into a dielectricmaterial, and a second semiconductor-element-containing dielectric layer31B.

Optionally, a mask layer (not shown) can also be used to define regionsin which at least one gate electrode (220A and/or 220B) is to berecessed, thereby limiting formation of recesses within a predefinedarea that is less than the entire area of a semiconductor circuitstructure. Such an areal limitation on formation of the recesses can beeffected by employing a hard mask (including a dielectric material suchas silicon oxide, silicon nitride, organosilicate glass, or acombination thereof) or a soft mask (a photoresist). The hard mask orthe soft mask can be removed after patterning of the recesses in desiredareas. For example, another gate electrode (not shown) can be formedconcurrently with formation of the first and second gate electrodes(220A, 220B) on the semiconductor substrate 8 such that a top metallicsurface of the other gate electrode is coplanar with the top surface ofthe planarization dielectric layer 60. Subsequently, the other gateelectrode is covered with a patterned mask layer, which does notoverlie, i.e., does not cover, the first and second gate electrodes(220A, 220B). Thus, the other gate electrode is not recessed while thetop surfaces of the first and second gate electrodes (220A, 220B) isrecessed relative to the top surface of the planarization dielectriclayer 60. A plurality of other gate electrodes (not shown) can be formedand covered by the patterned mask layer to prevent recessing. Thepatterned mask layer is subsequently removed, for example, by a dry etchor a wet etch or ashing. In some cases where a hard mask is employed,the hard mask could be kept in place.

Referring to FIG. 10, an etch stop dielectric layer 62 is formedcontiguously on the recessed top surface of the first gate electrode220A and the second gate electrode 220B and on the top surface of theplanarization dielectric layer 60. The etch stop layer 62 includes afirst portion 62A formed on the planarization dielectric layer 60 andsecond portions 62B formed on the first and second gate electrodes(220A, 220B). Each of the second portions 62B has a vertically offsetbottom surface relative to an interface between the planarizationdielectric layer 60 and the first portion 62A, i.e., the top surface ofthe planarization dielectric layer 60. The vertical offset distance isthe same as the recess depth rd.

The etch stop layer 62 can be formed by a conformal deposition, such aschemical vapor deposition (CVD), of a dielectric material that has adifferent composition than the planarization dielectric layer 60 and acontact-level dielectric layer to be subsequently deposited thereupon.In one embodiment, the etch stop layer 62 includes a material having achemical composition of SiC_(x)O_(y)H_(z) (also known as organosilicateglass), SiN_(w)C_(x)O_(y)H_(z) (i.e., nitrogen-doped organosilicateglass), SiC_(x)H_(z) (also known as BLoK™), or SiN_(w)C_(x)H_(z) (alsoknown as NBLoK™), wherein each of w, x, y, and z independently has avalue greater than 0 and less than 0.75. The thickness of the etch stoplayer 62 can be from 5 nm to 100 nm, and typically from 10 nm to 50 nm,although lesser and greater thicknesses can also be employed.

The first portion 62A and a center region of each second portion 62B ofthe etch stop layer 62 have a same composition and a same thickness. Thevertical recess over the first and second gate electrodes (220A, 220B)cause a peripheral region of each second portion 62B to have sidewallsthat are vertically coincident with sidewalls of a gate electrode (220Aor 220B). The peripheral portion of each second portion 62B extends froma level coplanar with a bottom surface of the center region of thesecond portion 6B to a level coplanar with a top surface of the firstportion 62A.

A contact-level dielectric layer 70 is deposited over the etch stoplayer 62. The contact-level dielectric layer 70 can be formed bydepositing a dielectric material, which is different from the dielectricmaterial of the etch stop layer 62. The dielectric material of thecontact-level dielectric layer 70 can be selected, for example, fromundoped silicate glass (USG), doped silicate glass, silicon nitride,organosilicate glass (OSG), undoped silicon oxynitride, and dopedsilicon oxynitride. If the contact-level dielectric layer 70 includes anorganosilicate glass and the etch stop layer 62 includes anotherorganosilicate glass, the compositions of the contact-level dielectriclayer 70 and the etch stop layer 62 differ so that the material of thecontact-level dielectric layer 70 can be etched selective to thematerial of the etch stop layer 62. The thickness of the contact-leveldielectric layer 70 can be from 30 nm to 300 nm, although lesser andgreater thicknesses can also be employed. Optionally, the contact-leveldielectric layer 70 can be polished in a planarization processing stepto facilitate subsequent formation of metal interconnect structures suchas contact vias and metal lines thereupon and/or to control thethickness of the contact-level dielectric layer 70 within apredetermined target range.

Referring to FIG. 11, a photoresist 77 is applied over the contact-leveldielectric layer 70 and lithographically patterned to form a pattern ofvia holes overlying contact regions of exemplary semiconductorstructure. The contact regions may include source and/or drain regionsof the first and second field effect transistor. In some cases, overlayvariations inherent in all lithography processes may cause peripheralregions of via holes in the pattern within the photoresist 77 to overliea portion of a gate electrode (220A and/or 220B) despite the conditionthat the intended contact areas are regions located on the top surfaceof the semiconductor substrate 8, such as the metal semiconductor alloyportions (46A, 46B).

The pattern in the photoresist 77 is transferred into the contact-leveldielectric layer 70 and subsequently into the etch stop layer 62 by atleast one etch that employs the photoresist 77 as an etch mask to formvia holes 65. For example, a first reactive ion etch that etches thedielectric material of the contact-level dielectric layer 70 selectiveto the dielectric material of the etch stop layer 62 can be employed totransfer the pattern in the photoresist 77 through the contact-leveldielectric layer 70. This segment of the etch process automaticallystops on the topmost surface of the etch stop layer 62 due to theselectivity to the material of the etch stop layer 62. Further, theexposure of surfaces of the etch stop layer 62 can be detected, forexample, employing an end point detection scheme that detects the changein composition of residual gases in the process chamber.

Upon detection of the end point for the etching of the contact-leveldielectric layer 70, the etch chemistry of the etch process can bechanged so that the dielectric material of the etch stop layer 62 isetched within the via holes 65. The etch stop layer 62 is etched throughunderneath the via holes 65 in areas overlying the planarizationdielectric layer 60, the first type stress-generating liner 58, and/orthe second type stress-generating liner 56, viz. in areas of the firstportion 62A (See FIG. 10). Because the upper surfaces of the etch stoplayer 62 are vertically recessed by the recess depth rd (See FIG. 10) inareas overlying a center portion of each gate electrode (220A, 220B),however, the recessed upper surfaces of the etch stop layer 62 are notexposed even after the etch stop layer 62 is etched through in areasoverlying the planarization dielectric layer 60, the first typestress-generating liner 58, and/or the second type stress-generatingliner 56.

Referring to FIG. 12, an additional etch process is performed to extendthe via holes 65 through the planarization dielectric layer 60, thefirst type stress-generating liner 58, and/or the second typestress-generating liner 56. The etch chemistry for the additional etchprocess is selected so that this etch is selective to the dielectricmaterials of the etch stop layer 62 and the first and second dielectricgate spacers (52A, 52B). Because of the selectivity of this etchchemistry to the dielectric material of the etch stop layer 62, the viahole 65 does not extend through the portions of the etch stop layer 62overlying the first and second gate electrodes (220A, 220B). Further,because of the selectivity of this etch chemistry to the dielectricmaterial of the first and second dielectric gate spacers (52A, 52B), thevia hole 65 does not expose the sidewalls of the first and secondreplacement gate structures (230A, 230B).

At the end of this etch, the via holes 65 do not vertically extend belowthe top surfaces of the remaining portions of the etch stop layer 62that overlie the first or second gate electrode (220A or 220B) becauseof the vertical offset between the first portion 62A and the secondportions 62B (See FIG. 10). Further, the via holes 65 do not laterallyextend beyond outer surfaces of the first and second dielectric gatespacers (52A, 52B). Thus, the via hole 65 at the end of this etch areself-aligned via holes, which are vertically self-aligned to a topsurface of the remaining portions of the etch stop layer 62 overlyingthe first and second gate electrodes (220A, 220B), and laterallyself-aligned to the outer surfaces of the first and second dielectricgate spacers (52A, 52B).

The original pattern in the photoresist 77 is modified by the shapes ofthe first and second replacement gate structures (230A, 230B) and thefirst and second dielectric gate spacers (52A, 52B). Some of the viaholes 65 can include a first via hole portion 63 that does not overlieany gate electrode (220A, 220B) and a second via hole portion 61 thatoverlies a gate electrode (220A, 220B). For each such via hole 65, thefirst and second via hole portions (63, 61) are contiguously connectedto each other. For each via hole 65 that overlies a portion of the firstand second replacement gate structures (230A, 230B), a first portion 61of the via hole 65 does not extend below a recessed portion of the etchstop layer 62 that overlies a gate electrode (220A, 220B). For such viaholes 65, a second portion 63 of the via hole 65 extends into theplanarization dielectric layer 60 (and the first type stress-generatingliner 58 and/or the second type stress-generating liner 56, if present)where the first and second gate electrodes (220A, 220B) and the firstand second dielectric gate spacers (52A, 52B) are not present. Thesecond portion 63 of such a via hole 65 can extend to a top surface ofthe semiconductor substrate 8, which can be a top surface of metalsemiconductor alloy portions (46A, 46B). The photoresist 77 issubsequently removed, for example, by ashing.

Referring to FIG. 13, self-aligned contact via structures 66 are formedby filling the via holes 65 with a conductive material and removing theportions of the conductive material deposited above the topmost surfaceof the contact-level dielectric material layer 70 by planarizationemploying, for example, chemical mechanical planarization and/or recessetch. The self-aligned contact via structures 66 extend through thecontact-level dielectric layer 70, the etch stop layer 62, and theplanarization dielectric layer 60, and if the first typestress-generating liner 58 and/or the second type stress-generatingliner 56 is/are if present, through first type stress-generating liner58 and/or the second type stress-generating liner 56. Each gateelectrode (220A, 220B) is spaced from the self-aligned contact viastructures 66 by a recessed portion of the etch stop layer 62 and adielectric gate spacer (52A or 52B). Thus, overlay variations during thelithographic process that defines the pattern of the via holes 65 do notresult in electrical shorts between the self-aligned contact viastructures 66 and the gate electrodes (220A, 220B).

In case a portion of a self-aligned contact via structure 66 in thecontact-level dielectric layer 70 overlies a gate electrode (220A or220B), the self-aligned contact via structure 66 is vertically spacedfrom the gate electrode (220A or 220B) by a recessed portion of the etchstop layer 62. Thus, a portion of the self-aligned contact via structure66 overlies the gate electrode (220A or 220B) and is vertically spacedfrom the gate electrode (220A, 220B) by the recessed portion of the etchstop layer 62.

Further, a lower portion of such a self-aligned contact via structure 66is laterally spaced from the nearest gate electrode (220A or 220B) atleast by a dielectric gate spacer (52A or 52B). Thus, each self-alignedcontact via structure 66 is spaced from the nearest gate electrode (220Aor 220B) by a recessed portion of the etch stop layer 62 and adielectric gate spacer (52A or 52B). A dielectric gate spacer (52A or52B) can laterally surround a gate electrode (220A, 220B) and can belaterally surrounded by the planarization dielectric layer 60. Eachelf-aligned contact via structure 66 can be spaced from the gateelectrode (220A, 220B) by the etch stop layer 62 and a dielectric gatespacer (52A or 52B). A U-shaped gate dielectric (32A, 32B; See FIG. 9)can contact a surface of the recessed portion of the etch stop layer 62.

Referring to FIG. 14, in an alternate embodiment, the formation of thecontact-level dielectric material layer 70 can be omitted and theself-aligned contact via structures 66 can be directly patternedemploying the combination of the etch stop layer 62 and a patternedphotoresist 77 at processing steps of FIGS. 11 and 12. In this case, aplasma RIE chemistry can be employed to form the via holes 65, andself-aligned contact via structures 66 are formed by filling the viaholes 65 with a conductive material and removing the portions of theconductive material deposited above the topmost surface of the etch stoplayer 62 by planarization employing, for example, chemical mechanicalplanarization and/or recess etch. In this case, self-aligned contact viastructures 66 are formed such that the topmost surfaces of theself-aligned contact via structures 66 are coplanar with the topmostsurface of the etch stop layer 62. Depending on the overlay variations,electrically floating metal portions can be formed above a gateelectrode (e.g., the second gate electrode 230B) and/or a self-alignedcontact via structure 66 can include an overhanging portion 66A, whichis an integral portion of the self-aligned contact via structure 66 andoverhangs above a gate electrode (e.g., the first gate electrode 230A).

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Accordingly, the disclosure is intended toencompass all such alternatives, modifications and variations which fallwithin the scope and spirit of the disclosure and the following claims.

1. A method of forming a semiconductor structure comprising: forming agate electrode and a planarization dielectric layer on a semiconductorsubstrate, wherein a top metallic surface of said gate electrode iscoplanar with a top surface of said planarization dielectric layer;recessing a top surface of said gate electrode relative to said topsurface of said planarization dielectric layer; forming an etch stoplayer contiguously on said recessed top surface of said gate electrodeand on said top surface of said planarization dielectric layer, whereinsaid etch stop layer includes a first portion located on saidplanarization dielectric layer and a second portion located on said gateelectrode and having a vertically offset bottom surface relative to aninterface between said planarization dielectric layer and said firstportion; forming a contact-level dielectric layer over said etch stoplayer; and forming a via hole extending at least through saidcontact-level dielectric layer, said first portion of said etch stoplayer, and a portion of said planarization dielectric layer, whereinsaid via hole is vertically spaced from said gate electrode by saidsecond portion of said etch stop layer.
 2. The method of claim 1,wherein said etch stop layer is formed by a conformal deposition so thatsaid first portion and a center region of said second portion of saidetch stop layer have a same composition and a same thickness.
 3. Themethod of claim 2, wherein said etch stop layer is formed so that aperipheral region of said second portion has sidewalls that arevertically coincident with sidewalls of said gate electrode, and extendsfrom a level coplanar with a bottom surface of said center region ofsaid second portion to a level coplanar with a top surface of said firstportion.
 4. The method of claim 1, wherein said etch stop layer isformed by depositing a dielectric material, wherein said dielectricmaterial has a different composition than said contact-level dielectriclayer and said planarization dielectric layer.
 5. The method of claim 4,wherein said etch stop layer is formed by depositing a dielectricmaterial having a chemical composition of SiC_(x)O_(y)H_(z),SiN_(w)C_(x)O_(y)H_(z), SiC_(x)H_(z), or SiN_(w)C_(x)H_(z), wherein eachof w, x, y, and z independently has a value greater than 0 and less than0.75.
 6. The method of claim 5, wherein each of said contact-leveldielectric layer and said planarization dielectric layer is formed bydepositing a material selected from undoped silicate glass (USG), dopedsilicate glass, silicon nitride, organsilicate glass (OSG), undopedsilicon oxynitride, and doped silicon oxynitride.
 7. The method of claim1, wherein said via hole includes a first via hole portion that does notoverlie said gate electrode and a second via hole portion that overliessaid gate electrode, and said first and second via hole portions arecontiguously connected to each other.
 8. The method of claim 1, furthercomprising forming a contact via structure by filling said via hole witha conductive material, wherein said gate electrode is spaced from saidcontact via structure by said etch stop layer and a dielectric gatespacer.
 9. The method claim 1, wherein said gate electrode is areplacement gate electrode formed by: forming a disposable gatestructure on said semiconductor substrate; and replacing said disposablegate structure with a gate dielectric and at least one conductivematerial, wherein said gate electrode is a remaining portion of said atleast one conductive material after removing said at least oneconductive material from above a top surface of said planarizationdielectric layer.
 10. The method of claim 9, further comprising: forminga dielectric gate spacer on sidewalls of said disposable gate structure;and forming a contact via structure through said contact-leveldielectric layer, said etch stop layer, and said planarizationdielectric layer, wherein said contact via structure is spaced from saidgate electrode by said etch stop layer and said dielectric gate spacer.11. The method of claim 9, wherein said at least one conductive materialincludes at least a titanium nitride portion and an aluminum portion,and said method includes: employing a wet etch solution includingammonium hydroxide and hydrogen peroxide to etch said titanium nitrideportion; and employing a solution including at least sulfuric acid andhydrogen peroxide or a dilute hydrofluoric acid-containing solution toetch said aluminum portion.
 12. The method of claim 9, wherein said atleast one conductive material includes at least a tantalum nitrideportion or a titanium nitride portion, and said method includesemploying a chemical downstream etch (CDE) to etch said tantalum nitrideportion or said titanium nitride portion, wherein said CDE employs atleast one of CHF₃, CF₄, and Cl₂ and does not generate plasma directly onsaid gate stack.
 13. The method of claim 9, further comprising: forminganother gate electrode concurrently with formation of said gateelectrode on said semiconductor substrate, wherein a top metallicsurface of said other gate electrode is coplanar with said top surfaceof said planarization dielectric layer; and covering said other gateelectrode with a patterned mask layer, wherein said patterned mask layerdoes not overlie said gate electrode, and wherein said other gateelectrode is not recessed while said top surface of said gate electrodeis recessed relative to said top surface of said planarizationdielectric layer.
 14. A semiconductor structure comprising: a gateelectrode located on a semiconductor substrate; a planarizationdielectric layer laterally surrounding said gate electrode, wherein atop surface of said gate electrode is recessed relative to a top surfaceof said planarization dielectric layer; an etch stop layer contiguouslylocated on said recessed top surface of said gate electrode and said topsurface of said planarization dielectric layer, wherein said etch stoplayer includes a first portion located on said planarization dielectriclayer and a second portion located on said gate electrode and having avertically offset bottom surface relative to an interface between saidplanarization dielectric layer and said first portion; a contact-leveldielectric layer overlying said planarization dielectric layer; and acontact via structure extending through said contact-level dielectriclayer, said etch stop layer, and said planarization dielectric layer,wherein a portion of said contact via structure overlies said gateelectrode and is vertically spaced by said second portion of said etchstop layer.
 15. The semiconductor structure of claim 14, wherein saidfirst portion and a center region of said second portion of said etchstop layer have a same composition and a same thickness.
 16. Thesemiconductor structure of claim 15, wherein a peripheral region of saidsecond portion has sidewalls that are vertically coincident withsidewalls of said gate electrode, and extends from a level coplanar witha bottom surface of said center region of said second portion to a levelcoplanar with a top surface of said first portion.
 17. The semiconductorstructure of claim 14, wherein said etch stop layer comprises adielectric material having a different composition than saidcontact-level dielectric layer and said planarization dielectric layer.18. The semiconductor structure of claim 17, wherein said etch stoplayer comprises a material having a chemical composition ofSiC_(x)O_(y)H_(z), SiN_(w)C_(x)O_(y)H_(z), SiC_(x)H_(z), orSiN_(w)C_(x)H_(z), wherein each of w, x, y, and z independently has avalue greater than 0 and less than 0.75.
 19. The semiconductor structureof claim 18, wherein each of said contact-level dielectric layer andsaid planarization dielectric layer includes a material selected fromundoped silicate glass (USG), doped silicate glass, silicon nitride,organsilicate glass (OSG), undoped silicon oxynitride, and doped siliconoxynitride.
 20. The semiconductor structure of claim 14, furthercomprising a dielectric gate spacer laterally surrounding said gateelectrode and is laterally surrounded by said planarization dielectriclayer, wherein said contact via structure is spaced from said gateelectrode by said etch stop layer and said dielectric gate spacer.